This invention relates to milling cutters and, more particularly to face milling tools primarily for metals.
A typical milling tool comprises a holder securable to the end of a rotating spindle in a predetermined relationship to the axis of rotation thereof and a short cutting edge carried by the holder at a radial distance from the axis of rotation on the side of the holder away from the spindle, so that as the body is rotated by the spindle, the cutting edge describes a circle about the axis of the spindle. All parts of the holder are axially and radially behind the cutting edge so that it is free to engage and machine the surface of a workpiece held in the machine in which the spindle rotates. Since the cutting edge is short, it takes a comparatively narrow cut and the tool is used with a transverse feed, i.e. the feed direction is parallel to the machined surface of the workpiece.
It has in the past been proposed to make a tool bit of wafer-like form for a variety of metal cutting tools. In other words, it is bounded by two generally flat parallel end faces and a side wall and has at least one cutting edge formed by part of the edge at the junction of the side wall and at least one of the end faces. It is of a thickness which is small compared to other dimensions and is typically secured detachably to a holder by means transversing a central aperture through it. If such a bit is of simple form and small size, it is cheap to produce and it is therefore economic to throw it away when blunt or worn rather than regrinding it, especially if it can be indexed to bring each of a plurality of cutting edge sections or cutting edges into cutting position.
In general terms, the two surfaces at the junction of which the cutting edge of a metal cutting tool is formed, are (a) the rake face against which the chip impinges and (b) the clearance face which is the face which is closer to the workpiece and which is inclined to the machined surface at a low angle which takes it out of contact with workpiece behind the cut. In the case of a wafer-like cutter as described above, according to the way it is located by the holder and the machine, it is possible for one end face to be the rake face and the side wall to be the clearance face, or for the side wall to be the rake face and the end face to be the clearance face. In the specific case of face milling cutters, nearly always a wafer-like cutter has been located so that the end face is the rake face and the side wall is the clearance face.
Nearly every milling operation consists of an interrupted cut. Normally, each cutting insert is in the cut less than half of the total machining time and, while the insert is in the cut, the thickness of the chip being formed constantly changes because of the dual motion, i.e. cutter rotation and workpiece feed motion, which is characteristic of the milling process. These features of milling operations result in the following cutting conditions:
1. As each insert enters the cut, it is subjected to a mechanical shock load. The magnitude of this shock load depends upon the workpiece material, cutter position, operating conditions and cutter geometry.
2. Cutting forces are cyclical, and are roughly proportional at any position in the cut to the undeformed chip thickness at that position. In a typical milling operation, undeformed chip thickness at insert entry is about 80% of the feed per insert, so forces begin high, build up gradually and finally peak as the insert crosses the feed axis, i.e. the point at which unformed chip thickness is at a maximum. Forces decline throughout the rest of the cut as undeformed chip thickness is constantly reduced. As the insert leaves the cut, forces drop to zero and remain there until the insert enters the cut again on the next spindle revolution.
3. Heat generated in the machining operation is also roughly proportional to the undeformed chip thickness. Thus, it is roughly proportional to the rapidly changing cutting forces. Such rapid changes in generated heat place a severe strain upon the cutting material and can lead to thermal cracking.
As an example of one of the previous milling cutters there can be mentioned the face milling tool described in U.S. Pat. No. 3,670,380, issued June 20, 1972. This tool utilizes a single face cutting insert of circular or lobed outline which is utilized essentially as a finishing insert. Thus, the disclosure of that patent points out that if the depth of material to be removed at one pass is greater than the insert can deal with, the holder can also carry one or more roughing bits which precede the circular or lobed insert.
It is highly desirable in certain operations to be able to use cutting inserts made from silicon nitride. This material is exceedingly hard, but it does present the problem that it is not possible to form a central aperture through the inserts for mounting to a holder.
It is an object of the present invention to provide a convenient means for mounting wafer-like cutting inserts on a holder without the need of apertures through the inserts.